This invention relates to a material in the form of a powder for a negative electrode for a non-aqueous electrolyte secondary battery which can reversibly occlude and release large amounts of alkali metals such as Li, and to manufacturing processes for the negative electrode material. This invention also relates to a process for manufacturing a negative electrode formed from this negative electrode material and to a non-aqueous electrolyte secondary battery using this negative electrode material and which is improved with respect to charge and discharge capacity as well as to cycle life.
A non-aqueous electrolyte secondary battery according to this present invention includes both batteries using a non-aqueous electrolyte in solution of a supporting electrolyte dissolved in an organic solvent, and batteries using a solid non-aqueous electrolyte in the form of a polymer electrolyte, a gel electrolyte, or the like.
As portable, small electric and electronic devices become more widespread and improve in performance, the production of non-aqueous electrolyte secondary batteries typified by lithium ion secondary batteries has greatly increased, and improvements in their capacity and cycle life are continuously demanded.
At present, in typical non-aqueous electrolyte secondary batteries, carbon materials are primarily used as negative electrode materials. However, in negative electrodes made from carbon materials, Li can only be occluded up to the composition LiC6. Therefore, the theoretical maximum limit on the capacity is 372 mAh/g, which is only approximately 1/10 that for the case for metallic lithium, and there is a limit on increases in capacity.
Metallic lithium, which was initially used as a negative electrode material, can provide a high capacity, but repeated charging and discharging of a battery cause the precipitation and growth of dendrite crystals, leading to the occurrence of short-circuiting, so the cycle life of charging and discharging was short, and it was not practical.
With the object of obtaining a high capacity, it has been proposed to use the element Al, which can reversibly occlude and release Li by the formation of an intermetallic compound, as a negative electrode material. However, due to changes in volume accompanying occlusion and release, the negative electrode material tends to form cracks which causes powderization or comminution of the material into a fine powder. Therefore, in secondary batteries using this negative electrode material, as cycles of charging and discharging progress, the capacity abruptly decreases, so they have a short cycle life.
As a measure to prevent this powderization of a negative electrode material caused by changes in volume, it has been proposed to add Li, Si, B, or the like to Al in a negative electrode material in order to increase the lattice constant of the aluminum material in advance (Japanese Published Unexamined Patent Application No. Hei 3-280363). However, the effect is inadequate, and it is not possible to sufficiently increase the cycle life.
It has also been proposed to occlude and release Li within the lattice of silicides or other intermetallic compounds (Japanese Published Unexamined Patent Applications Nos. Hei 7-240201, Hei 9-63651, etc.), but in each case, a significant effect was not obtained.
Various types of negative electrode materials for non-aqueous electrolyte secondary batteries and negative electrodes formed from those materials have been proposed, but a negative electrode material having a structure which can best exhibit the performance of those materials and a process for its manufacture have not been proposed.
An object of this invention is to provide a negative electrode material for a non-aqueous electrolyte secondary battery which can occlude and release large amounts of Li and which therefore, when used as a negative electrode material for a non-aqueous electrolyte secondary battery, provides a high charge and discharge capacity, a small decrease in capacity during repeated charging and discharging, and an excellent cycle life.
Another object of this invention is to provide a negative electrode material having a structure which enables a non-aqueous electrolyte secondary battery equipped with a negative electrode made from this negative electrode material to best exhibit its properties and to a manufacturing process for the negative electrode.
Silicon (Si) can reversibly occlude and release Li through the formation of an intermetallic compound with Li (such as Li22Si5). The charge and discharge capacity of Si when it is used in a negative electrode material for non-aqueous electrolyte secondary batteries is theoretically a high value of approximately 4020 mAh/g (9800 mAh/cc: specific gravity of 2.33). This theoretical maximum capacity is far larger than the theoretical maximum capacity of 372 mAh/g (844 mAh/cc: specific gravity of 2.27) of carbon materials which are actually used at present, and even compared with the theoretical maximum capacity of 3900 mAh/g (2100 mAh/cc: specific gravity of 0.53) for metallic lithium, it has a far larger electrode capacity per unit volume, which is important from the standpoint of reducing the size of batteries. Accordingly, Si can be used as a high capacity negative electrode material.
However, as is the case with Al, a negative electrode material made from Si metal easily turns to fine powder due to cracks formed by changes in volume accompanying occlusion and release of Li, so its capacity greatly decreases as charging and discharging cycles continue, and its cycle life is short. Therefore, up to the present, there have been almost no attempts using Si as a negative electrode material.
The present inventors noticed the high theoretical capacity of a negative electrode material made from Si. As a result of investigations aimed at increasing the cycle life thereof, they found that if the surface of Si phase grains is enveloped in a phase of an Si-containing solid solution or intermetallic compound, changes in volume accompanying occlusion and discharge of Li are restrained, so cracking and powderization of Si can be prevented, and the cycle life is increased. In order to sufficiently obtain this effect so that the restraint by the solid solution or intermetallic compound used for enveloping will be effective, the Si phase preferably has a small grain size. Such small Si phase grains can be efficiently formed by a rapid (cooling) solidification method.
The present invention provides a negative electrode material for a non-aqueous electrolyte secondary battery, which is made from alloy particles of a structure comprising one or more Si phase grains and a phase of an Si-containing solid solution or intermetallic compound which at least partially envelops the Si phase grains, wherein the average particle diameter of the alloy particles is at least 0.1 xcexcm and at most 50 xcexcm, and the Si phase grains constitutes at least 5 wt % and at most 99 wt % of the negative electrode material.
The xe2x80x9cSi-containing solid solution or intermetallic compoundxe2x80x9d which envelops the Si phase grains in the alloy particles can be constituted by Si and at least one element selected from the group consisting of Group 2A elements, transition elements, Group 3B elements, and Group 4B elements other than Si of the long form periodic table.
The negative electrode material for a non-aqueous electrolyte secondary battery can be manufactured by the processes described below.
One process comprises a step of cooling a melt of raw materials for forming alloy particles (elemental Si+at least one element capable of forming a solid solution or an intermetallic compound with Si) for solidification so as to obtain a cooling rate of at least 100xc2x0 C. per second, thereby forming an alloy comprising Si phase grains and a phase of an Si-containing solid solution or intermetallic compound which at least partially envelops the Si phase grains. This process may further include a step of subjecting the alloy obtained in the solidification step to heat treatment at a temperature at least 10xc2x0 C. below the solidus temperature of the solid solution or the intermetallic compound. The purpose of this heat treatment is to remove strains which develop within the alloy due to the rapid solidification. It can be carried out when the extent of strains is large. The cooling of the melt at a cooling rate of at least 100xc2x0 C. per second can be carried out by a method selected from the group consisting of the atomization method, the roller quenching method, and the rotating electrode method.
Another manufacturing process comprises a step of forming an adhering layer of a material which contains an element capable of forming a solid solution or an intermetallic compound with Si, on the surface of a powder of metallic Si or an alloy having an Si phase, and a step of subjecting the powder having the adhering layer to heat treatment at a temperature at least 10xc2x0 C. below the solidus temperature of the solid solution or the intermetallic compound to convert the material forming the adhering layer into an Si-containing solid solution or intermetallic compound. The adhering layer can be formed by a plating method or a mechanical alloying method
In either of the above-described manufacturing processes, it is preferable that the alloy particles which are formed contain on the average at least 5 wt % and at most 99 wt % of Si phase grains, and that the average particle diameter of the alloy particles be at least 0.1 xcexcm and at most 50 xcexcm.
According to this invention, a manufacturing process for negative electrodes for 3o non-aqueous electrolyte secondary batteries is also provided. This process is characterized in that a powder of metallic Si or of an alloy containing an Si phase is adhered atop a negative electrode substrate to form a powder layer on the substrate, thereafter a material comprising an element capable of forming a solid solution or an intermetallic compound with Si is plated on the powder layer, and then heat treatment is performed at a temperature at least 10xc2x0 C. below the solidus temperature of the solid solution or intermetallic compound such that the plating is converted into an Si-containing solid solution or intermetallic compound.
The present invention also relates to a non-aqueous electrolyte secondary battery equipped with a non-aqueous electrolyte and positive and negative electrodes which can reversibly occlude and release lithium, characterized in that the negative electrode contains the above-described negative electrode material or it contains a negative electrode material manufactured by either of the above-described processes for manufacturing a negative electrode material, or it is a negative electrode manufactured by the above-described process for manufacturing a negative electrode.
The negative electrode may contain at least 5 wt % and at most 80 wt % of a carbon material with respect to the above-described negative electrode material. The positive electrode preferably comprises a lithium-containing transition metal compound as an active material, and the non-aqueous electrolyte is preferably a solution of a lithium salt dissolved in an organic solvent containing a carbonate ester.